Those free-living polychaetes that evolved in the Cambrian
seas are the most ancient annelid group. From polychaetes, oligochaetes
evolved. Leeches are of oligochaete ancestry. Fossil
polychaetes have been found in rocks of the late Proterozoic eon
that contain the Ediacaran fossil assemblages (about 700 million
years old) and are well preserved in the middle Cambrian
rocks of the Burgess shale (500 million years old) of western
Canada. Terrestrial oligochaetes probably did not evolve before
the Cretaceous, when angiosperms, which contributed humus
in which earthworms live, arose. Annelids are possible ancestors
of sipunculans (A-24) and echiurans (A-25)—at least some
species of these phyla have trochophore larvae, produce gametes
from peritoneal tissue rather than ovaries and testes, and have
similar body-wall anatomy, nervous systems, excretory systems,
and patterns of gamete production. Immunological data support
annelids as closer relatives of sipunculans than are molluscs;
however, the evidence from paleontology, biochemistry,
and embryology supports molluscs as closer relatives of sipunculans
than are annelids. Data from ribosomal RNA indicate
that pogonophorans (A-25) may have evolved from annelids.